Unlike today's computers, these were truly "open" systems. All information about the computer was available - both hardware and software. Individuals were encouraged to design new and interesting items to add to the system. Prototyping boards for the NASBUS were readily available (and stripboard could be used at a pinch!)
Obvious third-party sales were of software but, unlike modern machines, most of the money was in retro-fitted hardware. This was because, at the time, advances in hardware were extremely fast. The basic machines had little capability as they came "out of the box" and needed quite a bit of expansion before they became really useful.
The first NASCOM computers had video displays designed for viewing on a standard TV set via a built-in RF modulator. (As a matter of interest, the 48 character 16 line display used by standard NASCOMs is about the maximum which can be handled by a standard TV tube - this is why many computers limit themselves to lines of 40 characters for TVs, it looks less cramped). Cost restraints limited them to mono displays. There was obviously going to be a large market for colour display systems if they could be manufactured at a low enough price. Hence there were IVC (Gemini), AVC (Lucas), Climax, SVC, Pluto (IO Research), VFC (Map) and possibly others - all with their own strengths. There were also various graphics adaptors for the NASCOM-1 from simple ROM changes to programmable character sets.
Other expansion was needed in the form of RAM and ROM as even the NASCOM-2 was quite limited in its basic form. Cases, cables, storage and I/O systems - all were produced in differing forms by competing companies - some very small indeed.